The invention generally relates to integrated circuit packages, and more particularly to a method and device for protecting an integrated circuit against attacks.
Integrated circuits (ICs) used in a number of embedded devices such as smartcards can contain a secret security key and carry out secret data. The IC needs to be secure against attacks from the outside.
Integrated circuits can be vulnerable to attacks on the physical structure of the integrated circuit device (such as a chip, semiconductor device, etc.).
Microelectronic technologies used to manufacture integrated circuits rely on a layer-based manufacturing process. During such process, material is deposited and etched to form stacked geometries which correspond to transistors, wires and layer-to-layer interconnections (also known as vias). Because this layered process relies on a silicon substrate to create the transistors, the first layers are always used to etch and deposit material related to transistor creation.
There exist different types of attacks against integrated circuits relying on physical modification of the integrated circuits. Such attacks are intended to gain information stored in the integrated circuit and/or to change the operating characteristics of the integrated circuits into other characteristics which can be exploited by the attacker.
ICs may be subject to front-side attacks. Because of the intrinsic structure of the IC manufacturing process, the first layers that are accessible to an attacker performing invasive attacks from the front side are the metal interconnect layers. Such metal interconnect layers form indeed a sensitive resource that an attacker may try to probe, modify or force to a specific value because they are responsible for transmitting valuable information from transistor to transistor.
Front-side attacks may consist of opening of packaged IC devices, and recording electrical signals from the IC device with external probes. To counteract front-side attacks, it is known to use a protection shield to prevent such attacks. The protection shield can be passive or active.
Passive shields can be used to prevent viewing of the circuit and making attacks more time-consuming. Passive shields may be removed without affecting the operation of the device. Passive shields are generally constituted of an upper layer of metal interconnects in a multi-layer circuit. However, a breach in the shield is not detected in passive shields.
Active shields are similar to passive shields. However, a breach in an active shield can disable the integrated circuit. Circumventing an active shield is possible theoretically but this is both complex and time-consuming while being limited to a small number of areas of the integrated circuit under attack.
The protection shield used to protect the IC front-side generally consists of metal structures on the top metal layers to prevent front-side invasive attacks. These structures involve designing a dense mesh that the attacker will need to cut through in order to access the sensitive information. Because of the microscopic nature of the devices, such operation involves additional costs for the attacker.
However, new forms of attacks, referred to as “backside attacks”, are emerging whereby the attack is not made through a front surface of the IC but through the silicon substrate via a back surface of the IC. Backside invasive intrusions that attempt to access valuable structures up from the silicon substrate were disclosed recently in “Breaking and entering through the silicon, Clemens Helfmeier and al., CCS 2013”. Backside attacks constitute serious threats.
Common shielding techniques are not adapted to prevent these types of attacks.
Backside attacks were elaborated to make circuit modifications to flip-chip devices or on lower metal layers of a multi-layer stacked IC device. These techniques are generally used in combination with invasive attacks such as wafer thinning, laser cutting and heating, focused ion beam (FIB) techniques.
As IC device designs comprise several layers, backside attacks try to reach a lower metal layer, for example, via the back surface rather than passing through many layers of interconnects from the front surface.
The conventional active shield used conventionally is arranged on the front surface of the IC device and is not adapted to prevent attacks conducted through the back surface via the substrate.
There is accordingly a need for improved methods and devices for protecting integrated circuits against backside attacks.